ANTIGENS

• Antigens are substances capable of inducing a specific immune response.

• Antigen – “Antibody + gen” (generator of antibody)

• Substances which when introduced into the animal body can stimulate immune response

• Antigen combines with specific receptors on the surface of antigen sensitive cells to stimulate a immune response.

• These cells multiply and differentiate into lymphocytes which can mediate a cellular immune response or into antibody producing plasma cells.

Definition of antigens based on immunogenic properties:

• Immunogenicity• Antigenicity• Allerogenicity• Tolerogenicity

• Immunogenicity: ability to induce humoral or cell mediated immune response.

• Antigen here is called an Immunogen

2. Two properties of immunogen

1. Immunogenicity

An ability of antigen which can stimulate the body to

evoke a specific immune response. 2. Immunoreactivity

An ability of antigen which can combine with

corresponding Ab or sensitized lymphocyte

III. Specificity and cross reaction of antigen

• Specificity is a cardinal feature of the adaptive immune system

• Specificity is referred to that immune responses are directed toward and able to distinguish between distinct antigen or small parts of macromolecular antigens.

• This fine specificity is attributed to lymphocyte antigen receptors that may bind to one molecule but not to another with only minor structural differences from the first

Specificity of Ag

Ab1 Ab2 Ab3

• Specificity exists in both immunogenecity and immunoreactivity

• Specificity is the basis of immunologic diagnosis and immunologic therapy as well as basic feature of adaptive immunity

1. Immunogen: the antigen that induce specific immune response

Microbes; bacteria ,virus; fungi and parasitesxenoantigeneic or allogeneic tissues or organs: grafted skin , bone marrow

• Antigenicity: Ability to combine with antibodies/cell surface receptors.

• All molecules that possess immunogenecity possess antigenecity but not vice-versa

• eg: Haptens possess antigenicity but cannot induce immune response.

• Allerogenicity: Ability to induce allergic responses.

• Allergens are immunogens that can activate specific type of humoral/cell mediated response having allergic manifestations.

3. Allergen: antigen that induce Anaphylaxis (severe immediate hypersensitivity reaction occurring as a result of rapid generalized mast-cell granulation)

Allergen: some medicine, flower powder, seafood

• Tolerogenicity:• Capacity to induce specific immunologic non-

responsiveness in either humoral or cell mediated branch

2.Tolerogen: antigen that induce Immunologic tolerance

• Immunologic tolerance is unresponsiveness to an antigen that is induced by prior exposure to that antigen.

tolerogen

Based on Chemical nature• Proteins

Majority of immunogens are proteins (pure proteins or they may be glycoproteins or lipoproteins). Proteins are usually very good immunogens.

• Polysaccharides Pure polysaccharides and lipopolysaccharides are good immunogens.

• Nucleic AcidsNucleic acids are usually poorly immunogenic. However, they may become immunogenic when single stranded or when complexed with proteins.

• LipidsIn general lipids are non-immunogenic, although they may be haptens.

Requirement of immunogenicity:Immunogenicity is not an inherent property but

depends on experimental conditions of the system. (which include –

the antigen, mode of immunization, organism immunised and sensitivity of methods used to detect a response)

• Foreigness: discrimination of self and non-self. Only foreign molecules should be immunogenic.

• Eg: albumin isolated from rabbit serum can be injected back into the same rabbit or another but will not generate antibody formation.

• Same injected in to any other higher vertebrates will evoke antibody production depending on the dose of antigen injected.

Molecular size

• Immunogenicity requires a specific minimum size.• Extremely small molecules like aminoacids or

monosaccharides are not immunogenic. • Mol wt of a ideal immunogen- approx 100,000da

• Very high molecule haemocyanin (MW 6.75million) –highly antigenic

• Low molecular weight molecule (<10,000) -non antigenic or feeble

They render antigenicity by absorbing inert particles like bentonite or kaolin

• Chemical complexity: A molecule must have a certain degree of

chemical complexity to be immunogenic. Immunogenicity increases with structural complexity.

• Genetic constitution of the animal:• Ability to respond to an antigen varies with

the genetic make up.• Pure polysaccharides are immunogenic when

injected into mice and human beings but not when injected into guinea pig.

• Method of antigen administration:• whether an antigen will induce an immune

response depends on the dose and mode of administration.

.

1. Antigen must be foreignness to immune system:

What substances are foreignness to immune system ?

According to Burnnet’s clone selection theory ,

foreignness ( non-self) means substances that never

contact with lymphocytes during embryo period.

What kinds of substances can be foreignness to immune system?

(1) Heterogeneous substances

Various pathogens, xenoantigeneic tissues

(2) Allogeneic substance

grafted allogeneic tissues or organs

(3)Autoantigenic components that never contact with lymphocytes during embryo period

Release of sequestered antigen------

Such as lens protein, sperm etc. Change of molecular structure of auto-tissue

For example, denatured IgG in patient suffering from rheumatoid arthritis becomes antigen to induce production of antibody ( rheumatoid factor)

• Antigens- synthetic or natural• Eg; bacteria, viruses or other microbes.• Foreign proteins – pollen, egg white.• Metabolic products of microbes and living cells from

different animal spp.• Protein or polysaccharides• Mol wt of > 10,000 daltons.• May be nucleoproteins, lipoproteins and glycoproteins

from any biological source or synthetic polypeptides or polysaccharides.

Degradability

• Antigens that are easily phagocytosed are generally more immunogenic. This is because for most antigens (T-dependant antigens) the development of an immune response requires that the antigen be phagocytosed, processed and presented to helper T cells by an antigen presenting cell (APC).

1. Antigen determinants (epitope)

(1) The portion of antigen molecules which can be

specifically recognized by antibody or antigenic receptor

of lymphocytes.

• Protein antigen----5-15 amino acid residues

• Polysaccharide antigen----5-7 polysaccharide residues

Chicken lysozyme bound to an antibody

Three dimension figure of Angiotensin(Ag) II binding to antibody

(2) A change of antigenic determinant (characteristics,

number and conformation) can influence the

specificity of Ag.

Antigen determinant is the sites of Ag combining with

Ab

The chemical component , arrangement and conformation affect the specificity of antigen

2. Antigenic valence

Total number of determinants which can be bound by

antibody or antigenic receptor of lymphocytes is

called antigenic valence.

Most natural antigens are polyvalence antigen.

3. Classification of antigenic determinant

(1)According to the structure of Ag determinants • Conformational determinants • Sequential (or linear) determinants

Conformational determinants

Conformational determinants

are formed by amino acid

residues that aren’t in a

sequence but become spatially

juxtaposed in the folded

protein.

Sequential (or linear) determinants

Epitopes formed

by several adjacent

amino acid residues

are called linear

determinants.

(2)According to types of cells recognizing antigenic determinants

• T cell determinants (T cell epitopes)

• B cell determinants (B cell epitopes)

Difference between T cell epitope and B cell epitope

T cell epitope B cell epitope

Receptor TCR BCRNature short peptide proteins, polysaccharides Size 8-17 amino acid residues 5-15 amino acid residues or 5-7 monosaccharidesTypes linear epitope conformational epitope or linear epitopePosition any position in antigen mostly exist on the surface of antigen

3.Common antigen and cross reaction

(1) Common antigen ( common determinants in fact ) Different bacteria which possess the same epitopes are

called common antigen. (2) Cross reaction ---Existence of common determinants Because there are some common antigen determinants existing in

different microbes, so the antiserum against one kind of microbe can also react with another microbe,this called cross reaction.

A1

2

3

2

B

Anti-typhoid

serum

Anti-Ag2

Anti-Ag1

Anti-Ag2

Anti-Ag3

Anti- Paratyphoid

serum

Typhoid bacillus

Paratyphoid bacillus H Ag

O Ag

flagellum

(3) Significance In clinic, existence of cross reaction may lead to

wrong diagnosis.

Flu virus typhoid

Complete antigen : substances with both immunogenecity and immunoreactivity

By convention , we call complete antigen as antigen.

Incomplete antigen (hapten): substances only with immunoreactivity

Hapten +carrier complete antigen (immunogens)

Hapten: Only possess immunoreactivity

Carrier: Make hapten obtain the immunogenicity

Based on Immunogenicity

According to source of antigens• Xenoantigen• Alloantigen • Autoantigen • Heterophile antigen

(1) Xenoantigen

• An antigen that is found in more than one species. An antigen is something that is capable of inducing an immune response.

• The prefix "xeno-" means foreign or other. It comes from the Greek "xenos" meaning stranger, guest, or host.

• Pathogens: bacteria, virus , fungi, parasite• Exotoxin and toxoid Exotoxin Produced by G+ bacteria Strong antigenicity and pathogenicity Toxoid : exotoxin that loses its toxicity but maintains its antigenicity under suitable

conditions (low concentration of formaldehyde ) Such as tetanus toxoid , diphtheria toxoid

HIV

Pathogens Fungibacteria

Heterophile Ag (forssman Ag) Common Ags shared by different species ( between human and

animal or microbes, between different species of microbe)

(eg) M protein of streptococus bears common antigen

determinant with basement membrane of kidney

(This common between bacteria and human being can causes

poststreptococcal glomerulonephritis)

No specificity of species

Significance . immunopathology

. Diagnosis

(2) Alloantigen/ isoantigen

A type of tissue specific antigen present in one individual but

not in other

Antigens of red blood cell

• ABO system (blood typing)

- very important in transfusion

• Rh system (Han race :>99%Rh+)----haemolytic disease of the newborn

(HDNB)

HLA system (Human leukocyte antigen)

- relate to transplantation

- very important in immune regulation

ABO system

Blood antigen antibody in typing of RBC serum A A anti-B B B anti-A AB A,B - O - anti-A, anti-B

(3) Autoantigen

Release of sequestered Ag

Lens protein is released into blood to induce immune response

to induce inflammation of lens

Change of molecular structure of auto-tissues

Denatured IgG becomes antigen to induce production of

antibody ( rheumotoid factor)

In patient suffering from rheumotoid arthritis

Exogenous Antigens

1- Bacterial antigens:

a- Antigens related to bacterial cells - Somatic antigen (O): part of cell wall gm –ve bacter. - Capsular antigen: usually polysaccharide - Flagellar Ag (H) : a protein made of flagellin - Fimbrial Ag: surface antigens in fimbriated bacilli

b- Antigen secreted by bacteria: - Exotoxins - Enzymes

2- Viral antigens: a- protein coat viral antigens b- Soluble antigens (soluble nucleoproteins as in influenza)

II. According to whether need the help of T cells when B cells produce Ab

TD-Ag (thymus dependent Ag )

TI-Ag (thymus independent Ag)

1.TD-Ag (thymus dependent Ag ) TD-Ag can stimulate B cell to produce Ab with the help of T cell

The most of TD-Ag belong to protein many kinds of determinants stimulate B cell to produce :IgG, IgM, IgA capable of inducing CMI immune memory

2. TI-Ag (thymus independent Ag)

TI-Ag can stimulate B cells to produce Ab without the help of T cell

most are polysaccharide more ,same, repeat determinant only induce B cell to produce IgM can not induce CMI no immune memory

SUPERANTIGENS

• When the immune system encounters a conventional T-dependent antigen, only a small fraction (1 in 104 -105) of the T cell population is able to recognize the antigen and become activated (monoclonal/oligoclonal response).

• However, there are some antigens which polyclonally

activate a large fraction of the T cells (up to 25%). These antigens are called superantigens

Eg: Staphylococcal enterotoxins (food poisoning), Staphylococcal toxic shock toxin (toxic shock syndrome), Staphylococcal exfoliating toxins (scalded skin syndrome) and Streptococcal pyrogenic exotoxins (shock).

The diseases associated with exposure to superantigens are, in part, due to hyper activation of the immune system and subsequent release of biologically active cytokines by activated T cells.

• Tumor specific Ag ( TSA) Only expressed on the tumor cells but normal cells

The tumor antigens encoded by genomes oncogenic virus

EB virus ---B cell lymphoma HPV-cervical carcinoma

• Tumor associated Ag (TAA)

Highly expressed on tumor cells but lowly expressed on normal cells, such as AFP CEA

AFP (alpha-fetoprotein): over-expression in liver cancer

CEA (carcinoembryonic antigen): over-expression in carcinoma of colon , pancreas, stomach ,and breast

Factors influencing Immunogenicty1-Foreigness : Foreign substances are immunogenic

2- Molecular size: High molecular weight increase immunogenicty

3- Chemical structure complexity: High complexity increase immunogenicty

4- Route of administration: Parenteral routes are more immunogenic to oral route

Factors influencing Immunogenicty

5- Method of administration: a- Antigen dose: Appropriate dose optimum antigenicty Low dose low- zone tolerance High dose high-zone tolerance

b- Adjuvant: Substance when injected with an antigen enhance immunogenicty